Circuit arrangement and methods for use in a wind energy installation

ABSTRACT

The invention relates to a circuit with a variable rotational speed to be used particularly in a wind power plant, comprising a double fed asynchronous generator (DASM), a crow-bar, an additional resistor (R 15 ) and a converter. In order to meet the requirements of the network provider, whereby a particularly permanent coupling to the network should be ensured so that the wind power plant can start up and stabilize the network during and after medium voltage short circuit in the network, the additional resistor can be regulated with the aid of a fast switch in such a way that the converter can be provisionally disconnected at least partly in case of a short circuit in the network. The rotor current is momentarily assumed by the additional resistor and disconnected after the rotor short circuit current dies out so that the converter can be subsequently connected once again and so that it can supply the desired active short circuit current to the network.

BACKGROUND OF THE INVENTION

The invention relates to circuit arrangement which is intended inparticular for use in a variable rotation speed wind energyinstallation, comprising a double-fed asynchronous generator, anadditional resistor and a converter; and methods hereof.

Circuit arrangements such as these which are intended for use invariable speed wind energy installations are widely used in practice andare thus prior art by virtue of obvious prior use. However, it has beenfound to be disadvantageous when using double-fed asynchronous machines(DASM) that these are disconnected from the network in the event of anetwork short circuit at the medium-voltage level. This means that it isnot possible to achieve the desired network stabilization by means of awind power installation which is operated with a double-fed asynchronousmachine.

In the past, the necessary network stabilization has thus been achievedby the network operators by means of conventional power stations. Owingto the rapidly increasing number of wind power installations and therapidly rising power level associated with them, which has now reachedthe magnitude of conventional power stations, the requirements for windpower installations must, however, be matched to those of conventionalpower stations. In particular, permanent network coupling is beingincreasingly demanded in order that the wind energy installation can setup the network again, and can stabilize it, once a medium-voltagenetwork short circuit has ended.

SUMMARY OF THE INVENTION

The invention provides a circuit arrangement for use in wind powerinstallations having an asynchronous machine, by means of which morestringent requirements for modern wind power installations, inparticular with regard to network stabilization, can be satisfied.

According to the invention, this is achieved by circuit arrangements andmethods according to the features of the independent patent claims. Thedependent claims relate to particularly expedient embodiments of theinvention.

Thus, according to the invention, a circuit arrangement is proposed inwhich the additional resistor can be controlled by means of a high-speedswitch such that the converter can be at least temporarily switched offin the event of a network short circuit, in order for the rotor currentto be taken over in the short term by means of the additional resistor,and can be connected to the network again for active injection of ashort-circuit current after the rotor short-circuit current has decayed.

This allows the more stringent network requirements for networkstabilization during operation of the wind power installation equippedwith an asynchronous generator to be optimally satisfied, because nodisconnection from the network takes place in this case in the event ofa network short circuit at the medium-voltage level. For this purpose,for example, an additional resistor which is in the form of acontrollable load resistor or a crow bar which is equipped with theadditional resistor for this purpose was inserted in the rotor circuitto draw the rotor short circuit energy when a network short circuitoccurs, and which is then switched off once the short-circuit currenthas decayed. The load resistor is controlled by a switch which, inparticular, can be actively switched off and is, in particular, not anaturally commutated thyristor. The existing rotor inverter for thefour-quadrant inverter is briefly deactivated immediately after theoccurrence of the network short circuit and is activated again after theshort-circuit equalization process has decayed, with the threshold valueadvantageously being below the rotor inverter rated current, and thenfeeds the necessary power into the network during the network shortcircuit and when the network voltage returns.

A modification of the present invention has been found to beparticularly advantageous in this case, in which the circuit arrangementhas two or more resistors which can be connected dependent on oneanother or independently of one another. This means that the high rotorshort-circuit current, which is frequently more than 1000 A, can beshared between a number of switches, since these switches which can beswitched off would have to be connected in parallel in a highly complexmanner for the total current.

A circuit arrangement having a two-point regulator for control of theadditional resistor is also particularly advantageous since this allowsvery simple, high-speed and robust control to be set up.

In this case, a further modification has been found to be particularlyexpedient in which the active switch is controlled by pulse-widthmodulation at a fixed clock frequency, because this allows digitalcontrol at a fixed clock frequency.

Furthermore, it also promises to be particularly successful for theactive switch to be controlled by a P regulator, a PI regulator or a PIDregulator. This means that the rotor short-circuit current or the rotorterminal voltage can be optimally regulated when a network short circuittakes place.

A refinement of the circuit arrangement according to the invention isalso particularly advantageous in which in the event of a network shortcircuit, a capacitive current or an inductive current is supplied to theshort circuit, since this allows the network to be stabilized in anoptimum manner depending on the network operator requirement. Acapacitive current is normally desirable in order to supply theinductive network loads.

It is also particularly worthwhile to prevent any wattless componentfrom being transmitted into the short circuit when a network shortcircuit occurs since this results in the least current being fed intothe short circuit, in order to avoid overloading existing medium-voltageswitches.

Furthermore, according to a further particularly advantageousrefinement, an additional impedance is briefly inserted in the statorcircuit in order to limit the stator and the rotor current. Theinsertion of the additional impedance as required allows the statorcurrent and rotor current to be limited when the network voltagereturns.

An embodiment is also particularly advantageous in which a high-seedcontactor is inserted in the stator circuit in parallel with theadditional impedance, in order in this way to bridge the additionalimpedance during normal operation, and to avoid producing losses.

Furthermore, it also promises to be particularly successful for at leastone thyristor with natural commutation to be inserted in the statorcircuit in parallel with the resistor. Compared with switches which canbe switched off actively, this results in reduced losses during normaloperation, with the costs being lower.

Furthermore, the circuit arrangement can be designed in a particularlyadvantageous manner by operating a controlled resistor on theintermediate circuit of the converter, because this makes it possible tosave some of the components in the crow bar, and the control for therotor inverter measures the rotor phase current all the time.

Another particularly expedient refinement of the invention is alsoachieved by operating a controlled resistor both in the crow bar and inthe intermediate circuit of the converter. This allows power sharing,and smaller individual switches can be used. Towards the end of theequalization process for the rotor short-circuit current, all of therotor current is carried, and the rotor inverter control then measuresthe entire phase current.

Furthermore, a particularly advantageous embodiment of the invention isalso achieved by switching off the rotor inverter when the networkvoltage returns, with the overcurrent then being carried by thecontrollable resistor, in order to actively carry the rotor current oncethe overcurrent has decayed and the controlled resistor has beenswitched off. This avoids the wind energy installation from possiblybeing switched off and disconnected from the network, in particular whenthe network voltage returns suddenly.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention allows various embodiments. One of these is illustrated inthe drawing, and will be described in the following text, in order tofurther explain the fundamental principle of the invention. In thefigures:

FIG. 1 shows a circuit arrangement according to the invention;

FIG. 2 shows one possible short-circuit profile;

FIG. 3 shows a circuit arrangement with a controllable rotor resistanceand an additional stator resistance;

FIG. 4 shows a voltage- and current-time profile with an additionalresistor;

FIG. 5 shows a circuit arrangement with stronger inverter diodes and acontrollable load resistance in the intermediate circuit.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a circuit arrangement according to the invention. Duringnormal operation, a switch V15, for example an IGBT, GTO, IGCT, isswitched off and the crow bar is fully inactive. All of the rotorcurrent flows into a converter and is regulated by it. If a networkshort circuit occurs at the medium voltage, the full excitation of anasynchronous generator means that it will supply an equalizationshort-circuit current to the short circuit. The current is limited onlyby the stray inductances of the asynchronous generator andmedium-voltage transformer, and the maximum current reaching thefollowing value:

$I_{stator} \approx {1.8 \cdot \frac{U_{network}}{{Xtr} + {X\; 1} + {X\; 2^{\prime}}}}$

In this case, Xtr is the total stray impedance of the transformer, X1 isthe stray impedance of the stator and X2′ is the stray impedance of therotor. In the event of a short circuit on the medium voltage, themaximum stray current is in practice in the order of magnitude of up to8 times the stator rated current. The rotor current is coupled bytransformer action to the stator current and also reaches up to 8 timesthe rotor rated current. This high equalization current cannottechnically sensibly be carried or absorbed by the converter. When theshort circuit occurs, a rotor inverter is switched off due to theovercurrent. The rotor current continues to flow via freewheeling diodesin the rotor inverter, and charges an intermediate circuit C3. At thesame time, the voltage across a capacitor C10 in the crow bar rises.When the voltage across the capacitor C10 reaches a limit value, theswitch V15 is switched on. A resistor R15 carries all of the rectifiedrotor current, and the voltage across the capacitor C10 falls below thevoltage limit value, so that the switch V15 is switched off. The voltageacross the capacitor C10 then rises again owing to the rotor current,and the switch V15 is switched on again. The rate of current change andhence also the clock frequency are governed by L15. The clock frequencyis up to the kHz range and cannot be produced by natural commutation ofthyristors, since the maximum rotor frequency is 15 Hz. This two-pointregulation results in a constant back e.m.f. for the rotor voltage, andthe equalization current decays in a very short time because of the highconstant back e.m.f. All of the current is commutated from the rotorinverter to the crow bar. The converter current is virtually zero. Thecrow bar current is measured and evaluated by the control board. Theload resistance is designed for maximum current, and the time for whichthe switch V15 is switched on is initially close to 100%. As theequalization current falls, the time for which the switch V15 isswitched on becomes less and is approximately 12% at the rotor ratedcurrent, which corresponds to approximately ⅛ of the maximum current. Itwould also be feasible to use two or more resistors, which can beconnected and disconnected individually. When the equalization currentfalls below the rotor rated current, then the switch V15 is switched offcompletely, and the rotor current commutates back into the converter.The converter starts to operate and provide regulation, and activelyfeeds the short circuit. While the controllable resistor is active, thenetwork inverter can be switched off, although simultaneous operation isalso possible. For safety reasons, a thyristor V10 is provided in thecrow bar, which automatically measures the voltage and is triggered inthe event of failure in the switch V15 or in the event of a directgenerator short circuit. L10 prevents the current from risingexcessively fast, in order to prevent destruction of the thyristor V10.In this case, D10 prevents rapid discharging of a capacitor C10 throughthe switch V15. The switch V15 can be controlled either directly in thecrow bar or by the control board for the converter.

FIG. 2 illustrates one possible short-circuit profile, with the dashedline representing the medium voltage and the solid line representing thenetwork voltage. The short circuit occurs at the instant 0 ms. Thecurrent immediately jumps to the maximum value and then decays, as aresult of the equalization process. The high current is drawn by thecrow bar and resistor. When the current falls below the rotor ratedcurrent, it is once again transferred to and controlled by theconverter. The generator is overexcited and supplies a capacitivewattless component to the network during the network short circuit.However, inductive current can also be fed into the short circuit. Thiscan be preset freely as required. Owing to the voltage drop across themedium-voltage transformer, the network voltage is in the order tomagnitude of about 20% of the rated voltage. At the instant when thevoltage returns, the voltage does not rise suddenly to the rated value,but rises over a dU/dt flank. A dynamic overcurrent occurs in the statorand rotor as a result of the flank gradient of the returning networkvoltage and the time constant of the generator. It must be possible forthis overcurrent to be supplied by the converter without this leading tothe rotor inverter being switched off. If the flank gradient is toogreat or there is a phase fault between the generator voltage and thereturning network voltage, then the dynamic overcurrent or equalizationcurrent will be excessively high, and the rotor inverter is switchedoff. In this case as well, the controllable resistor briefly carries theequalization current and, once the current has fallen below the rotorrated current, the resistor is switched off and the rotor inverter isonce again regulated. The controllable resistor is briefly activatedduring the drop in voltage and when the voltage returns. The rotorinverter is switched off during this time.

In the event of extremely fast voltage rise times, an additionalimpedance, for example in the form of a resistor or an inductor, can beinserted in the stator circuit. A system such as this is illustrated inFIG. 3. A contactor K20 is inserted between the medium-voltagetransformer and the generator/converter system. A resistor R20 isconnected in parallel via the contactor K20. If a short circuit occurs,then the contactor K20 is opened and the stator current flows throughthe resistor R20.

FIG. 4 shows the voltage-time profile with an additional resistor. Thestator current is limited and decays more rapidly than with only theregulated crow bar. The contactor has to switch very quickly for theresistor to be active in the event of very short voltage drops. Aback-to-back parallel-connected thyristor switch with naturalcommutation may also be used with, for example, a switching off time of6.7 ms at 50 Hz. This results in a high-speed switch, that has thedisadvantage of high losses, compared with the contactor solution. InFIG. 4, the switch is opened after 10 ms. The converter once againprovides the control function after the equalization process. Owing tothe additional voltage drop across the resistor, the residual networkvoltage is higher than that without any additional impedance in thestator. When the voltage returns, the additional resistor limits thedynamic stator current rise, thus allowing higher voltage flanks andlower overcurrents.

The freewheeling diodes of IGBT modules are not designed for very highpulse currents. The components of the controlled resistor were thereforeplaced in the crow bar. FIG. 5 shows a circuit arrangement withhigh-power freewheeling diodes. The switch V15 is coupled directly tothe intermediate circuit of the converter, and directly regulates theintermediate voltage. This would simplify the entire design. Theadditional standard crow bar is retained for extreme situations.

It would also be possible to completely dispense with the crow bar. Inthis case, the additional resistor must be designed for all extremesituations. In the event of a short circuit, the rotor inverter IGBTsare switched off, and the rotor short-circuit current flows through thefreewheeling diodes into the intermediate circuit. If a limit value isexceeded, the additional resistor is activated, and the short-circuitenergy is absorbed in the additional resistor. Once the short-circuitcurrent has decayed, the rotor inverter is activated once again, and theadditional resistor is switched off. It is also possible to switch theadditional resistor off first of all, and to connect the rotor inverter.Simultaneous operation of the additional resistor and of the rotorinverter is also possible.

1. Circuit arrangement which is intended in particular for use in avariable rotation speed wind energy installation, comprising adouble-fed asynchronous generator, an additional resistor and aconverter, characterized in that the additional resistor can becontrolled by means of a switch such that the converter can be at leasttemporarily disconnected in the event of a network short circuit, inorder for the rotor current to be taken over in the short term by meansof the additional resistor, and can be connected to the network againfor active injection of a short-circuit current after the rotorshort-circuit current has decayed.
 2. Circuit arrangement according toclaim 1, characterized in that the converter can be connected below arotor inverter rated current once the rotor short-circuit current hasdecayed.
 3. Circuit arrangement according to claim 1, characterized inthat the circuit arrangement has two or more resistors which can beconnected dependent on one another or independently of one another. 4.Circuit arrangement according to claim 1, characterized by a two-pointregulator for controlling the additional resistor.
 5. Circuitarrangement according to claim 1, characterized in that the switch iscontrolled by pulse-width modulation at a fixed clock frequency. 6.Circuit arrangement according to claim 1, characterized in that theswitch is controlled by a P regulator, a PI regulator or a PIDregulator.
 7. Circuit arrangement according to claim 1, characterized inthat the additional resistor is a component of a crow bar.
 8. Circuitarrangement according to claim 7, characterized in that the activeswitch is controlled directly by the crow bar.
 9. Circuit arrangementaccording to claim 8, characterized in that the crow bar switch iscontrolled directly by the converter control board.
 10. Circuitarrangement according to claim 1, characterized in that, in the event ofa network short circuit, a capacitive current or an inductive current issupplied to the short circuit.
 11. Circuit arrangement according toclaim 1, characterized in that, in the event of a network short circuit,no wattless component is transmitted to the short circuit.
 12. Circuitarrangement according to claim 1, characterized in that an additionalimpedance is briefly inserted in the stator circuit in order to limitthe stator and rotor current.
 13. Circuit arrangement according to claim12, characterized in that a high-speed contactor is inserted in thestator circuit in parallel with the resistor.
 14. Circuit arrangementaccording to claim 12, characterized in that at least one thyristor withnatural commutation is inserted in the stator circuit in parallel withthe resistor.
 15. Circuit arrangement according to claim 1,characterized in that a regulated resistor is operated on theintermediate circuit of the converter.
 16. Circuit arrangement accordingto claim 7, characterized in that a regulated resistor is operated bothin the crow bar and in the intermediate circuit of the converter. 17.Circuit arrangement according to claim 1, characterized in that, whenthe network voltage returns, the rotor inverter is switched off, and theovercurrent is passed through the controllable resistor and, once theovercurrent has decayed, the controlled resistor is disconnected and therotor inverter actively carries the rotor current.
 18. Circuitarrangement for use in a variable rotation speed wind energyinstallation comprising a double-fed asynchronous generator, anadditional resistor and a converter, wherein the rotor inverter isswitched off when the network voltage rises, and the overcurrent ispassed through the controllable resistor and, once the overcurrent hasdecayed, the controlled resistor is switched off and the rotor inverteractively carries the rotor current.